=DATEDIF(A1,B1,"y")
gives the time difference in years between a date in A1 and a date in
B1
so the formula
=DATEDIF(A1,DATE(2006,7,31),"y")
with a date of birth in A1 gives age at 31st July 2006.
For next year change the year to 2007 or put your date in a cell, e.g
B1 and use
="U-"&DATEDIF(A1,B1,"y")